A conventional quantum encryption communication apparatus utilizing a heralded single-photon source transmits a heralding signal output from the single-photon source from a transmitting apparatus to a receiving apparatus. In the transmitting apparatus, a quantum signal modulating operation is performed by using the heralding signal as a trigger. Also, in the receiving apparatus, a photon detecting operation and a quantum signal demodulating operation are implemented by using the transmitted heralding signal as a trigger (e.g., refer to non-patent documents 1 and 2).
A heralded single-photon source is a system that generates a twin-photon pair by using a parametric down conversion and the like, confirms, by measuring one photon of the pair, a presence of the other photon of the pair without having to measure it, and uses it as a single-photon source. Here, a measurement output of one photon of the pair that has been measured is output as a heralding signal notifying an output of the other photon of the pair. As a pump light source generating the parametric down conversion and the like, a continuous wave (CW) laser (e.g., non-patent document 1) or a pulse laser is utilized. In either one, a twin-photon pair is generated as a probabilistic generation phenomenon. Hence, in the way of a photon source, a single photon ends up being generated at an irregular time interval.
A reason why the heralded single-photon source is preferably used in quantum encryption is because it is a light source having less probability of generating a multi-photon state and having greater single-photon property than a conventional one that reduces laser light. When a typical photon detector currently being utilized in quantum encryption is used, a security cannot be assured over a communication distance of about 25 km (kilometers) if the laser light is utilized as the single-photon source. On the other hand, for a case of the heralded single-photon source, a security is assured even at a distance exceeding 50 km since it has greater single-photon property.
Specifically, in the heralded single-photon source, in general, by reducing an intensity of the pump light, two-photon presence probability P(2) for an optical pulse specified by the heralding signal can arbitrarily be made small while maintaining one-photon presence probability P(1). Therefore, a security of the quantum encryption similar to the case of using an ideal single-photon source can be realized.    Non-patent document 1: A. Trifonov and A. Zavriyev, “Secure communication with a heralded single-photon source,” Journal of Optics B: Quantum Semiclass, Opt. 7 No 12 (December 2005) S772-S777, 23 Nov. 2005    Non-patent document 2: S. Fasel, O. Alibart, S. Tanzilli, P. Baldi, A. Beveratos, N. Gisin and H. Zbinden, “High-quality asynchronous heralded single-photon source at telecom wavelength,” New Journal of Physics 6 (November 2004) 163, 12 Nov. 2004.